In maritime commerce ocean-going cargo vessels are found to have an optimum draft for most efficient carriage of cargo. The more a cargo vessel can be loaded up to its optimum draft, the more efficient its usage. As more cargo is carried, unit costs are lowered and the more effective overall maritime commerce becomes. This is basically because over a wide range of ship loadings, the costs of operating a cargo vessel is essentially a function of the calendar days it is in operation and the number of sea miles that it travels.
The use of ocean-going ships to transport cargoes to maritime ports via shallow channels, canals, or rivers or in enclosed harbors, presents a restriction on the overall operational capabilities of an ocean-going cargo vessel. It is commonly found that many cargo vessels are capable of being utilized at drafts well in excess of the maximum depth within the exisiting maritime channels on inland waters in the United States. This has lead to extensive, expensive long term engineering projects which attempt by dredging or by other means to artificially maintain river channels at depths suitable for full ocean-going vessels. Such dredging is expensive and requires constant work. In addition, it introduces significant environmental hazards into coastal marshlands and wetlands areas, especially by permitting salt water intrusion further up into fresh water estuaries than otherwise would naturally occur. This salt water intrusion has been known to cause serious damage to coastal waters' ecology, especially in regards to the disturbance of the habitat for many valuable maritime species in commerce such as shrimp and oysters.
Methods of temporarily reducing the draft of vessels have been suggested, primarily within the area of maritime salvage. They generally take the form of buoyant bags or bladders, interconnected by cables and nets, which are placed under the hull of a vessel and which are used to lift it clear of the water. The resulting conglomeration essentially is unpropellable due to extensive drag and the lack of any steerage way or control.
Alternative forms of buoyancy reduction have been developed within the area of drydocks, in which large, relatively complicated structures are used to raise ships totally clear of the water level. Such structures have severe strength restrictions because they are also designed to support ship repair activities, and thus they are generally only capable of raising a ship which has been emptied to its lowest displacement weight. In addition they are static structures; they are not intended to propell or to otherwise permit the motion of a ship while it is raised to the more buoyant position.